Electric discharge device



April 19, 1932. c. A. SABBAH ELECTRI C DISCHARGE DEVICE File d Feb. 28,- 1951 Inventor Camil A. Sabbah, b W66 His Attorne 1 1.

Patented Apr. 19, 1932 untras'r'res CAMIL A. SABIBAI-I, 0F SCHENECTADY, NEW YORK, ASSIGNOB TO GENERALIELECTI BIC COMPANY, A CORPORATION OF NEW YORK ELECTRIC DISCHARGE DEVICE Application filed February 28, 1931. Serial No. 519,202.

My invention relates to vapor electric apparatus such as the mercury arc rectifier or inverter, particularly to the shield or control electrode devices or the like associated with the anodes of such apparatus, and has for its object the provision of an improved anode shield or control electrode device whereby the occurrence of cathode spots thereon is prevented while permitting the device to assume a desired potential.

In the operation of the mercury arc rectifier and like apparatus it is customary to provide metallic members interposed between the anodes and cathode, these members functioning as anode shields or are guides, or as control electrodes or grids, or simultaneously as anode shields and control electrodes. The members may be so mounted as to surround the associated anode except for an opening permitting the arc discharge to pass, or may be mounted below or adjacent the anode in the path of the arc discharge between anode and cathode.

Difficulties have been encountered, however, in the use of these metallic shield or grid members. In many instances for example steel shields have been found to be perforated and heavily corroded I discharge passing through the material of the shield instead of around its edge to the anode, this action of the discharge formlng on the shield a cathode spot which by reason of the condensation upon the associated anode of the metal vapor thereby formed caused are back of this anode. In order to avoid this arcing difliculty encountered in the use of steel or other metallic shield or grid members it has been proposed to form these'memin operation by an arc member are retained at the same time that the disadvantages thereof are obviated, by the provision of a shield or grid member composed of a material through which the discharge currents causing formation of cathode spots and consequent arcing in the metal type of shield or grid member cannot flow, but whose surface nevertheless is'capable of assuming a definite potential. Toattain this result I provide a shield or grid member preferably constituted by a body of insulating material covered with a thin film of conductive material. A shield or grid member thus formed will have an equipotential surface but will not permit an arc of appreciable inten sity .to discharge on any point of its surface since the current supplying such an arc is limited entirely to the current which can flow in the surface layer. As this layeris of extremely high resistivity, being preferably a;

metallic film of molecular order of thickness, the surface current is of insuflicient magnitude to start or sustain a cathode spot.

My invention will be. better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claimsf Referring to the drawing, the single figure thereof is a fragmentary sectional viewof an electric discharge device in which my invention has been embodied. I y v In the figure is shown a mercury arc rectifier or inverter 1 comprising an evacuated tank 2, a mercury cathode 3 and a plurality of anodes 4 of which, for clearness of illustration, only one is shown in the figure. In the path of the arc discharge between cathode 3 and anode 4- is mounted in any suitable mannera member 5 having a wall portion 6 with which the arc discharge comes in contact. In the present embodiment of the invention the member 5 is shown as an anode shield or are guide. It is to be understood, however,

that this member 5 or other similar, membersi of suitable form mounted in any suitable manner may be used in accordance with my invention for other purposes, particularly as control electrodes or grids. V V y In order to prevent an arc discharge from passing through the wall portion 6 of the member instead of around the edge 7 thereof to the anode 4 the wall 6 comprises a body of insulating material 8. In order at the 5 same time to permit the surface of the member 5 to assume a desired potential, the inf sulating material 8 is coated with a thin conducting film 9 preferably metallic, of high resistivity to prevent flow of current over the surface sufficient to cause the formation of cathode spots. 7

In operation of the rectifier or inverter 1, the metal coated insulating member 5, while preventing the occurrence of cathode spots and consequent are back, will acquire any potential applied at any point of its surface 9 as effectually as a like member formed entirely of metal. If the metal coated member 5 is used as an anode shield or arc guide theare starting difficulties encountered in the use of shields composed entirely of insulating material are therefore obviated, and if used mit the charging thereof to a predetermined potential and sufficiently resistive to prevent a substantial flow of current thereover.

7 In anelectric discharge device comprising an anode and a vaporizable cathode, a member disposed in the arc discharge 'between said electrodes, said member including a body of insulation to prevent the flow there through of current tendingto form a cathode spot thereon and including a film of current conducting material on the surface of as a control electrode, any desired control I potentials may be impressed by any suitable and well known means upon the metal surface 9.

Further, "in operation of the rectifier or I inverter 1, the shield or grid member, comprising as described an insulating'body 8 and a conductive coating 9 thereon, is immune to damage by are backs under all conditions of arc drop and of vacuum, since the current necessary to form a'cathode spot or an are on the member 5 cannot be supplied because of the high resistance of the surface layer.

What I claim as new and desire to secure by Letters Patent of the United States, is,

'1. An anode shield member or the like including a wall of insulating material having a surface capable of being charged to a predetermined potential. i

2. An anode shield member or the like composed of insulating material covered with a thin layer of conductive material.

3. An anode shield member or the like comprising a body of insulation covered with a metallic film.

4. An electric discharge device including a cathode, an anode, and a member mounted in the path of the arc'discharge between said electrodes, said member being provided with a high resistance current-conducting surface.

' 5. An electric discharge device including a an anode and an insulating shield member or the like associated with said anode, said member being provided with a high-resist-' ance conductive surface layer. I

6. An electric discharge device including a cathode, an anode and a member disposed in the arc discharge between said electrodes, said member including a wall having suf ficient resistance to prevent substantial flow of current therethrough, the surface of said member being sufliciently conductive to per- 

